Searching for dark matter in the Earth

Many experiments search for dark matter either directly, by trying to detect dark matter particles themselves for the first time, or indirectly, by trying to measure the effects of dark matter self-annihilation. The latter is the case for IceCube, which has already searched for dark matter by looking for a neutrino flux that would be created in the annihilation of dark matter particles in the Sun, in the Milky Way, or even in nearby galaxies.

Now the IceCube Collaboration has expanded these studies with a search for dark matter annihilations in the center of the Earth. Researchers have used one year of data—May 2011 to May 2012—and have not found an excess of neutrinos above the expected background. The results have set new limits on the annihilation rate of WIMPs in the Earth that are an order of magnitude stronger than previous results by AMANDA and that also improve the IceCube spin-independent cross section limits for a WIMP mass of 50 GeV. This study has just been submitted to The European Physical Journal C.

Dark matter - earth
Upper limits at 90% confidence level as a function of the WIMP mass. Systematic uncertainties are included. The result is compared to the limits set by SuperCDMSlite, LUX,

Observations of the universe have given us many hints of the presence of a huge amount of dark matter. The rotational curves of galaxies and the distribution of the cosmic microwave background, just to mention a couple of examples, suggest that the matter we can see is only a small fraction of all the matter in the cosmos.

Indirect searches for the most promising dark matter candidates, such as the so-called WIMPs, look for the signal of secondary particles produced by self-annihilating dark matter. Among those, neutrinos are the only ones that can probe massive baryonic bodies like the Sun or the Earth, as they alone provide irrefutable testimony of a hadronic interaction.

Dark matter particles from the galactic halo can accumulate in the center of the Earth. First, they would be gravitationally trapped by the solar system while traveling through the galaxy. Then, their weak interactions with nuclei in the Earth would result in an energy loss and an accumulation in Earth’s core.

The self-annihilation of this dark matter would produce a signature of neutrinos that would depend on the density of the accumulated dark matter, the specific annihilation channel, and the WIMP mass. The chemical composition of the Earth and the dark matter model would also impact the expected neutrino rates.

All these uncertainties provide scenarios where the neutrino flux can fluctuate between 10-8 and 105 neutrinos per square kilometer per year for WIMPS with masses in the GeV-TeV range. AMANDA and other experiments have already ruled out fluxes above approximately 103. The increased size of IceCube now allows searching for lower fluxes. The null results improved upper limits by an order of magnitude, and new IceCube data will push these limits even further.

The results of this search are complementary to other results of direct searches, indirect searches, and searches at colliders,” explains Jan Kunnen, who worked on this research as a graduate student at Vrije Universiteit Brussel. “The current analysis is sensitive to both the dark matter capture in the Earth, and thus the spin-independent scattering cross section, as well as to the WIMP annihilation cross section. Thus, our results provide a natural bridge between different dark matter experiments, which in general are sensitive to only one of these quantities,” adds Kunnen.

+ info “First search for dark matter annihilations in the Earth with the IceCube Detector,” IceCube Collaboration: M. G. Aartsen et al. European Physical Journal C77 (2017) 82,